The present invention relates generally to fuse assemblies for protecting against electrical surges and/or overcharging. More particularly, an improved externally heated positive temperature coefficient (PTC) fuse assembly is described.
In a number of electrical components, it is desirable to provide fuses to protect against electrical surges. There are currently a number of commercially available fuse styles. One available type of fuse is known as a positive temperature coefficient fuse. A representative positive temperature coefficient (PTC) fuse is illustrated in FIG. 1A. The representative PTC fuse 50 has a top plate (top contact pad) 52 and a bottom plate (bottom contact pad) 54 that are separated by a positive temperature coefficient resistor (PTC) material 56. The PTC fuse 50 operates such that, at a threshold temperature, the electrical resistance of the PTC material 56 increases dramatically.
As seen in the battery protection diagram 60 of FIG. 1B, the PTC fuse 50 may be used to protect a battery 75 during charging with a voltage source 77. By way of example, the battery 75 may be included in a cellular telephone and the PTC fuse 50 may be implemented to protect the battery 75 from excessive current as a result of an individual using an improper voltage source, e.g. an improper power pack.
The contact pads 52 and 54 each have tabs 57, 59 which are bent down to form sidewalls that define a cavity 61 beneath the bottom contact pad 54. Solder posts 67 are formed on the tips of bent down tabs 57, 59. With this arrangement, the fuse can be mounted on a printed circuit board 69 (or other appropriate substrate) by soldering the solder posts 67 to appropriate landings 71 on the printed circuit board 69.
In operation, the PTC material 56 often does not heat up fast enough to protect the battery 75. As illustrated in FIG. 1C, in order to expedite heating of the PTC material 56, a die 64 may be attached to the bottom surface 55 of the bottom contact pad 54 to externally heat the PTC material 56. The die 64 may include a flip chip packaged zener diode, for example, which generates heat once a particular voltage has been reached. The die 64 also includes solder bumps 66 to mount the die 64 on the board 69 by soldering the solder bumps 66 to appropriate landings 79.
For the battery protection diagram 80 of FIG. 1D, the threshold voltage for the packaged zener diode of the die 64 is then set relative to the voltage of the battery 75. When the threshold voltage of the packaged zener diode is exceeded, it begins to draw current and externally heat the PTC material 56. The die 64 material (i.e., silicon) typically heats up more rapidly than the PTC fuse 50, and with the contact area between the die 64 and the bottom contact pad 54, the PTC material 56 reaches its threshold temperature faster. In addition, when the threshold voltage of the packaged zener diode is exceeded, the diode conducts at lesser resistance than the battery 75 and PTC fuse 50. As a result, the current is shunted away from the battery 75, further protecting the battery 77 and increasing the heat generation in the die 64.
In some cases, upon heating of the die 64, the solder bumps 66 may begin to melt. In other cases, upon heating of the die 64, the die 64 thermally expands and the solder bumps 66 crack. In either case, contact is lost, the die 64 is no longer functional and the protection circuit is unreliable.
In view of the foregoing, it should be apparent that an improved PTC device would be desirable.
To achieve the forgoing and other objects and in accordance with the purpose of the invention, an improved fuse assembly for protecting against electrical surges is described. The fuse assembly includes a PTC fuse, a die that serves as an external heat source for the PTC fuse and a lead frame based structure that facilitates electrical connection of the fuse assembly to a suitable substrate. The PTC fuse includes a PTC material which is normally conductive, but upon reaching a predetermined temperature, becomes non-conductive or substantially less conductive. To expedite heating of the PTC material, the fuse assembly includes a die package in thermal communication with the PTC material. The die package includes a die which generates heat once a threshold has been reached. To mount the die package to a substrate, the die package includes leads. The leads may be bent and surface mounted to the substrate using conventional techniques. The leads are sufficiently flexible such that the heat generated by the die does not compromise performance of the PTC protection circuit, i.e. by thermal expansion of the die package or parts thereof.
In a described embodiment, the improved PTC fuse assembly is used to protect a battery. In this embodiment, the die is placed in electrical parallel to the terminals of the battery and PTC fuse such that when a predetermined threshold voltage is reached, the current shunts through the die. In one embodiment, the die consists of one or more zener diodes which heat up after the predetermined threshold voltage has been reached. Correspondingly, the PTC material is externally heated, thereby activating the PTC fuse, which further prevents undesired current flow to the battery.